Sometimes, I load a YouTube video on my browser, it won’t play the audio but the video plays normally.
Probably you have a problem with you audio drivers you can reinstall or update them, also sometimes the browser isn't working properly all you need to do is restart it or try another browser.
well does that particular youtube video has audio?
How do I disable or enable hardware acceleration?
Yes. That video has audio, it's just that the video is not transmitting audio to my speakers, for whatever reason.
it can be because the audio driver, try to reinstall it
Check this link : https://support.google.com/youtube/bin/answer.py?hl=en&answer=58132
"Check any Antivirus and Firewall software to see if its blocking third-party Flash content."
How do I do this step? I use Bitdefender Total Security 2013.
Bitdefender Total Security 2013 User Guide
see if mute button is pressed on their accidentally
Unfortunately when I raise the volume of the video (not of the computer), still no sound comes out.
hmm then are you sure that your drivers are properly installed I mean if other sounds are playing well ..